Printing apparatus and method for saving printing material

ABSTRACT

A method and apparatus for printing rasterized images while economizing printing resources, on the basis of input image data corresponding to a predetermined original format, are provided. The method includes reducing the input image relative to the original format; on the basis of the reduced-scale image, forming a raster of image points each defining either a point to be printed or a blank point of the rasterized image; enlarging the raster of points to return to the original format by inserting blank image points in the reduced raster by a predetermined filling procedure; and printing the image on the basis of the point raster enlarged to the original format.

This application is the national phase under 35 U.S.C. §371 of prior PCTInternational Application No. PCT/IB97/00243 which has an Internationalfiling date of Mar. 12, 1997 which designated the United States ofAmerica, the entire contents of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of printing raster imageswhile economizing printing material, starting from input image datacorresponding to a predetermined original format and comprising the stepof forming a raster of image points in which the rows and columns arereduced by a factor n with regard to the raster in the standard mode andin which each image point defines either a point to be printed or ablank point of the rasterised image, for use with color or monochromeprinters that operate by means of a raster, such as ink jet printers orlaser printers, and also thermal or electrostatic printers. Theinvention also relates to a printer of the above-specified type enablingsuch a method to be implemented.

2. Discussion of Background Art

With raster-printing printers, a matrix of elementary points is createdin memory on the basis of input image data. Each matrix point is indexedto a determined point of the print output raster. It has a binary valuethat determines whether the corresponding point in the raster is to belinked (a printed point or “dot”) or left white (a blank point).

Input image data is generally of three types: vectors, gray-levelrasters, and binary rasters.

Data in vector form defines lines to be printed by specifying a startpoint and an end point in a coordinate plane.

Data in gray-level raster form defines a mosaic of image points in amatrix that is configured in rows and columns, where each element has anintensity value allocated thereto in the range of white to black.

Data in binary form is made up of rasters as in the preceding case, butthere are only two possible values for each point of the image,corresponding either to a point that is printed as a dot or to a pointin the raster image that is left blank. A half-tone appearance can berendered by modulating the density of printed points.

In certain applications, it is often necessary to print several draftsfor visual inspection before printing a final version. For printingdrafts, it is common-place to use draft-mode printing, i.e. printing arelatively low print density so as to reduce the running cost of theprinter and also extend the number of sheets it can print.

Numerous known printing methods exist for reducing consumption ofprinting material in an economic print mode.

For example, it is proposed in document JP-A-52 61973 to define a printzone in the form of a matrix of image points over which a patterned maskis superposed electronically to prevent certain points being printed,depending on the pattern.

In another method described in document JP-A-62 212 164, alleven-numbered points in consecutive runs of image points to be printedare replaced by blank points in the row direction.

In next another method described in document EP-A-625 765, a maskpattern is generated which is applied to the image data prior toprinting to reduce the number of dots actually printed.

In next another method described in document EP-A-582 434, the rasterdata are depleted in horizontal direction (dot reduction) and the headscans the row at higher rate, so the printed size of a page is notchanged.

In methods of those types, based on eliminating in a systematic mannersome of the points that are to be printed there is the risk ofcompletely deleting from the printed output certain patterns defined byfine rows or by isolated points.

Other known methods operate by selectively eliminating points forprinting by taking account of the immediate graphics environment, inorder to reduce the risk of unwanted deletion.

For example, document EP-A-689 159 proposes to form a raster of imagepoints in which the rows and columns are reduced by a factor n withregard to the raster in the standard mode each image point definingeither a point to be printed or a blank point of the rasterized image,which raster is logically combined with a set of three-pixel bits heldboth in vertical and horizontal direction with the pixel of interestbeing located in the center, to perform dot reduction on the basis ofdetected edge patterns.

Document U.S. Pat. No. 5,390,290 proposes an ink-saving print method inwhich one point out of every three points to be printed is eliminatedfrom a matrix of points according to the following rule: for three, notnecessarily consecutive, points to be printed, the first point isretained, and the second and third points are eliminated and replaced bya new point situated halfway between the two points that are eliminated.

In another method described in document U.S. Pat. No. 5,270,728, seekingin particular to avoid unnecessary overlap between printed dots, everyother point is eliminated in each consecutive run of points to beprinted along each row, with odd rows being scanned in one direction andeven rows in the opposite direction.

There also exist printing methods which seek more specifically to reduceprinting time, either by spacing out dots or by eliminating a fractionof them using criteria analogous to those described above.

All of those known methods eliminate image points from the matrix ofbinary points formed from input graphics data. However, the processwhereby a matrix of binary points is formed, particularly from graphicsdata in the form of vectors or rasters of gray levels, itself gives riseto a certain amount of degradation of image information, since itproceeds by approximation. Eliminating points from the matrix can onlyaccentuate that degradation of information.

As a result, during printing, prior art techniques lead all too often tofine rows or to sets of isolated points being lost. However, it isimportant to be able to reproduce patterns of that kind reliably, evenin print economy mode. This applies particularly to images having a highconcentration of details carrying technical information, in particularin engineering, e.g. for printing electronic circuit layouts.

Furthermore, determining which points to eliminate from the matrix ofpoints requires considerable amounts of computation time and of memoryspace, which adversely affects printing time.

The object of the invention is to make it possible, in a draft printmode, to print images with considerable savings of printing material,while ensuring that all of the essential elements in the supplied imagedata are reproduced, the method also being fast, and sparing in memoryspace requirements.

To this end, the method according to the preamble is characterized inthat it furthermore comprises the following successive steps:

enlarging said raster of points to return to the original format byinserting blank image points in the reduced raster by a predeterminedfilling procedure; and

printing the image on the basis of the point raster enlarged to theoriginal format.

It will be observed that the method of the invention makes it possibleto operate on an image of reduced size, thereby reducing both theprocessing time necessary and memory capacity required for the step offorming the point matrix.

The savings in printing material come from the reduction of the inputimage which, since it has a format that is smaller in terms of imagearea, makes it possible to obtain a corresponding reduction inconsumption of printing material. Energy savings are particularlysignificant with thermal printers.

Reducing the input image does indeed lead to an inevitable loss ofdefinition. However, the image retains the essence of its informationcontent, thereby making it possible to maintain sufficient image qualityin the output print, e.g. to verify its graphics content and layout.

Also, the method does not require points for printing to be eliminatedfrom the point matrix, as is the case with conventional printingtechniques for economizing print. As a result, the graphics informationis better restored in the printed image.

The input image is advantageously reduced by a scale factor of n, wheren is a real number.

For a relatively simple implementation of the invention, the scalefactor n can be an integer equal to or greater than 2, and preferablyequal to 2.

When the scale factor n is an integer, the step of enlarging the rastermay consist an integer equal to or greater than 2, and preferably equalto 2.

When the scale factor n is an integer, the step of enlarging the rastermay include adding n-1 blank image point(s) between two points in eachrow of image points in the reduced raster, and n-1 row(s) of blankpoints between two rows of image points in the reduced raster.

When the input data is in the form of vectors specified in a coordinateplane, the reduction step may include dividing the definition of theinput image by n.

When the input data is in the form of a raster, the reduction step mayinclude forming a reduced raster in which each point is given arepresentative value based on the data in a neighborhood zone of theinput raster and having topographical correspondence with the point.Several techniques that are known per se may be employed for thispurpose.

In a preferred embodiment of the invention, this neighborhood zone isconstituted by an n×n block of points constituted by n contiguous imagepoints in the row direction and n contiguous points in the columndirection of the input raster.

When the input raster is a gray-level raster, the representative valuemay be an average of the gray levels in the neighborhood zone.

When the input raster is a binary raster in which each point has one orthe other of two values defining a point to be printed and a blankpoint, the representative value is advantageously derived from thepoints in the neighborhood zone in which such a manner as to avoideliminating fine patterns. When the neighborhood zone is a 2×2 pointblock, the representative value corresponds, for example, to a point tobe printed if half or more of the points in the block are points to beprinted, and otherwise it corresponds to a blank point. It ispreferable, however, for the representative value to be calculated so asto give the representative value the value of a point to be printedwhen:

half or more of the points of the block define a point to be printed; or

one point only of the block is a point to be printed and the block isthe N.m-th block in a run of not-necessarily consecutive blocks eachcontaining only one point to be printed, where the blocks are counted byany systematic counting techniques, where N is a run of integers 1, 2,3, . . . , and where m is a predetermined integer such that the reducedraster includes one point to be printed for each set of m input rasterblocks containing only one point to be printed in each block, in orderto avoid systematically eliminating isolated points to be printed in theinput raster.

Under such circumstances, the number m is preferably equal to 4.

This variant is particularly effective when the binary input rasterscontain numerous isolated points for printing which would otherwise runthe risk of being systematically eliminated. It often happens that inputbinary image rasters are themselves generated by image processing inwhich a half-tone appearance is generated by space modulation of thepoints to be printed (dithering), which gives rise to rather a largernumber of isolated points for printing.

Using the values n=2 and m=4, one in four of the isolated points in thebinary raster will still be printed. It is thus possible to maintain aprinting material saving of 75% while retaining a sufficiently largenumber of isolated points for them to reproduce satisfactorily thehalf-tone appearance of the original image.

For color printing, the steps of reducing the image, of forming theraster of points, and of enlarging the raster can be performed for eachcolor component, and possibly also the black component, of the image tobe printed.

The invention also provides apparatus for printing raster images whileeconomizing print, based on the above method.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and the advantages that stemtherefrom will appear more clearly on reading the embodiments describedbelow by way of non-limiting indication and with reference to theaccompanying drawings, in which:

FIG. 1 is a highly diagrammatic representation of a print head of anink-jet printer according to an embodiment of the present invention;

FIG. 2 is a diagram showing a portion of the ink-jet printer includingthe print head of FIG. 1;

FIG. 3A shows a vector pattern in a coordinate plane and as defined byinput image data according to an embodiment of the present invention;

FIG. 3B shows the FIG. 3A vector pattern after a scale reduction by afactor of 2;

FIG. 3C shows a matrix of binary points formed from the reduced vectorpattern of FIG. 3B;

FIG. 4A shows a gray-level graphics pattern according to an embodimentof the present invention;

FIG. 4B shows a raster defined by the input image data corresponding tothe gray-level pattern of FIG. 4A;

FIG. 4C shows the gray-level raster of FIG. 4B after a scale reductionby a factor of 2;

FIG. 5A shows a binary raster according to an embodiment of the presentinvention;

FIG. 5B shows the binary raster of FIG. 5A after a scale reduction isperformed in accordance with an implementation of the invention;

FIG. 6 is a block diagram showing the main steps of transforming inputimage data for printing without reducing the number of points to beprinted;

FIG. 7 is a block diagram showing the main steps of transforming theinput image data for printing while economizing the printing material,in application of the present invention;

FIG. 8A is an example of an image printed on an ink-jet printer in aprint mode without reduction of the number of points to be printedaccording to an embodiment of the present invention; and

FIG. 8B is an example of an image that was originally identical to thatof FIG. 8A but printed while economizing the printing material inapplication of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Before explaining the print method of the present invention in detail,the characteristics of an ink-jet printer with which the method can beused are described briefly, it being recalled that the method can beimplemented with other types of printers or devices equipped withprinting functions, in particular laser printers, electrostaticprinters, or thermal printers.

FIGS. 1 and 2 are diagrammatic representatives respectively of a printhead 10 of an ink-jet printer, and of the portion of the printer inwhich the head 10 is located.

The print head 10 comprises a line of nozzles 11, each capable ofejecting independently and in controlled manner a droplet of ink onto aprint medium 12, e.g. a sheet of paper. The print head 10 is movable inan X direction along a beam 14 which extends transversely relative tothe sheet 12. The sheet is displaced in a Y direction perpendicularly tothe X direction. The line of nozzles 11 in the print head extends in theY direction. It may alternatively be slightly inclined relative to thisdirection.

An image is reproduced by scanning successive strips B of width L bymoving the head 10 along the X direction (main scan). During a scanpass, ink ejection is controlled to reproduce the desired image in theform of ink dots. After each pass, the number of rows of points is equalto the number of nozzles 11, and definition in the Y direction isdetermined by the pitch p of the nozzles. After each pass, the sheet 12is advanced in the −Y direction (secondary scan) and the print head 10is caused to perform a new scan pass.

The displacements of the print head 10 and of the sheet 12, and theejection of ink are all performed by means that are well known and whichneed not be described herein.

The input image data can be encoded in vector form, in gray-level rasterform, or in binary raster form. The main characteristics of each ofthese forms of encoding are briefly explained below.

FIG. 3A is a graphical representation of an example of data in vectorform. The vectors are defined by the coordinates of their start and endpoints in an X, Y coordinate plane.

The vectors are written in a matrix of individual cells, each having aspecific address given by its (X, Y) coordinates, the matrix beingindexed relative to a print zone on the medium. The values X and Y arethe incremental values of the abscissa and ordinate coordinates. Theincrement between two adjacent cells fixes the maximum possibledefinition that can be printed from input data. A set of vectors isrepresented mathematically by the expression {(X0i, Y0i), (X1i,Y1i),i=1l} where 0 is the start point, 1 is the end point, and i is thevector index.

In the example, of FIG. 3A, crosses in some of the cells indicate vectorstart or end points. The coordinates of some of the points are given inparentheses. The vectors may also define a single point in thecoordinate matrix to cause a single dot to be printed at thecorresponding point of the medium. In this case, the start and endpoints of the vector coincide (isolated crosses in FIG. 3A).

FIG. 4B shows data encoded in the form of a gray-level raster from thepattern of FIG. 4A. The raster is constituted by addressable imagepoints configured in rows and columns. Each point is given a value 0 to15 as a function of its darkness (in the example, the value “0”, whichcorresponds to a blank point, is not shown). Each image pointcorresponds to a predetermined point to be printed on the print medium.The set of image points of the raster, which constitutes a map of theimage of FIG. 4A, serves as a reference for printing.

It will be observed that the unit increment between two adjacent pointsof the raster fixes the maximum possible print definition.

FIG. 5A shows data encoded in binary raster form. The encoding is thesame as for a gray-level raster, except that only two values arepossible for each image point: “1” or “0”; corresponding respectively toa point to be printed or to a blank point. In FIG. 5A, raster imagepoints having the value “1” are printed black. Naturally, the abovespecified correspondence can be inverted, depending on the protocolselected.

With reference to FIG. 6, there follows an explanation of the main stepsof printing at a normal inking density with the printer operatingconventionally, as a function of the above three types of input graphicsdata.

When the graphics data is in the form of vectors, the vectors areconveyed to a binary rastering unit 20. The unit 20 subdivides the (X,Y) coordinate plane (FIG. 3A) into image points or pixels by giving eachcell in the (X, Y) coordinate plane a binary value. In the example, theunit 20 gives the binary value “1” to cells in the coordinate matrixthat are crossed by a vector, and also to those containing a vectorstart point or end point. All other cells of the coordinate matrix aregiven the binary value “0”. The method of transforming vectors into abinary raster is known. The Breshenam algorithm is often used for thispurpose.

The binary values calculated in this way are stored in a point matrixmemory 22 where they are indexed in a manner that maintains thetopography and the scale of the vector coordinate matrix. To this end,the point matrix memory 22 defines an array of rows and columns of cellsthat correspond to the extracted binary raster.

Cell data is read from the point matrix memory 22 row by row into abuffered memory 24, from which it is transmitted in the form of printdata to print control units.

The sequence of data along each row of the point matrix memorycorresponds to a sequence of print points in the X direction of the head10, and the sequence of data in each column corresponds to a sequence ofprint points in the Y direction of the medium 12 (FIGS. 1 and 2).

When the input data is in the form of a gray-level raster, the raster isinitially converted into half-tones by processing in a half-tone unit 26so as to be transformed into a binary raster having the same scale asthe original raster in terms of number of image points. Methods forconverting gray-level rasters into binary rasters are well known. Forexample, it is possible to use space modulation which includes definingareas formed by groups of binary points, each area containing a numberof points to be printed that is a function of the gray level to bereproduced.

The points of the binary raster formed in this way are transmitted tothe point matrix memory 22 from which they are subsequently read as inthe preceding case.

When the input data is already in the form of a binary raster, it istransmitted directly to the point matrix memory 22.

An implementation of the printing material-economizing print method ofthe invention is explained below as a function of each of the threetypes of graphics data and with reference to the block diagram of FIG.7.

In this figure, there can be seen the binary rasterizing unit 20 usedfor data input in vector form, the unit 26 for converting a gray-levelraster into a binary raster, the point matrix memory 22, and the buffermemory 24. The operation of these units is the same as in the case ofFIG. 6, and it is therefore not described again.

Similarly, consideration is given to three types of graphics input datathat are identical to the preceding cases, taking as examples the vectorpattern of FIG. 3A, the gray-level raster of FIG. 4B, and the binaryraster of FIG. 5A.

When the data representing the input image for printing with ink economyis in vector form, the vectors are subjected to a scale reduction by ann factor. In this example, the scale is reduced by a factor of 2 (by ascale reduction unit 28). Such scale reduction is a conventionaloperation which includes dividing the coordinates of the start and endpoints of each vector by the reduction factor, which is equivalent todividing the definition of the input image by the reduction factor. Thevectors are then reconstructed on the original coordinate matrix on thebasis of these new coordinates. Starting from the example of FIG. 3A,vectors reduced in accordance with this method are shown in FIG. 3B.

Thereafter, the rasterizing unit 20 forms a binary raster on the basisof the vectors at reduced scale. The way in which “1” or “0” values areallocated to the image elements in the binary raster is identical to theway this is done for vectors in conventional print mode, and cantherefore be based on the Breshenam algorithm. The binary rastercorresponding to the reduced scale vectors is shown in FIG. 3C. Comparedwith the binary raster obtained from the vectors at the original scale,this raster is reduced by a factor of 2 in rows and in columns, and thetime required to compute the raster is also reduced by a factor of 2.

The reduced scale raster is transmitted to the point matrix memory 22from which it is transferred row by row into the buffer memory 24.

However, in order to restore the original image format on printing, thereduced scale raster is reconfigured by adding blank points in the rowsand columns. For this purpose, the following rules may be adopted:

a blank point is added between two points in each row of the reducedbinary raster; and

a row of blank points is added between two rows of points in the reducedbinary raster.

In the example, blank points are inserted between two points in each rowwithin the buffer memory 24. To this end, the buffer memory 24 receivesa sequencer of “0” binary data from a “0” value insertion unit 30. Thisunit 30 is controlled by a scale-restoring control unit 32 so that a “0”value is inserted alternatively with each point along each row.

For example, consider an isolated sequence S1 of eight bits: . . .11011101 . . . in a row of the point matrix stored in the buffer memory24, the unit 30 transforms the sequence S1 into the following sequenceS2: . . . 1010001010100010 . . . by inserting “0” after each bit of theS1.

Rows of blank points are inserted between each pair of rows downstreamfrom the buffer memory 24 by a “0” value row insertion unit 34 forinserting rows of value “0” and a switch 36, controlled by thescale-restoring control unit 32. The switch 36 has two inputsrespectively receiving the output from the buffer memory 24 and theoutput from the unit 34 for inserting lines of value “0”, and it has oneoutput from which print data is taken.

The switch 36 selectively transmits one of its two inputs at a frequencywhich is twice the row transfer frequency from the buffer memory 24. Asa result, the print data output from the switch 36 includes one row of“0” value points for each row of data in the buffer memory 24. In theabove example, the sequence S2 is thus transformed into a data block S3covering two line:

. . . 0000000000000000 . . .

. . . 1010001010100010 . . .

It will be observed that the final configuration of the print data doesnot depend on the order in which blank rows and columns are inserted. Itis also possible to envisage other algorithms for inserting “0” valuesto expand the configuration of the data in the point matrix so as toreturn to the original scale of the input image.

For example, for an image that has been reduced by a factor 2, it ispossible to insert blank points in a staggered row or column pattern inthe raster to be printed. The raster data stored in the memory 24 isthen staggered in rows or columns in the raster to be printed using acomplementary pattern.

Compared with normal print mode, the number of points having the value“1” (corresponding to points to be printed) is reduced by about 75% whenthe input image is reduced by a factor equal to 2. This results in acorresponding reduction in the quantity of printing material used.

When the input image data is in gray-level raster form (FIG. 4B) thescale of the data is initially reduced, e.g. by a factor of 2 (n=2), ina gray-level raster reduction unit 38. This unit 38 creates a raster atreduced scale by giving each point in this raster a value representationof the data in a block of four points in the input raster, each blockcomprising two contiguous image points in the row direction and twocontiguous image points in the column direction thereof. Techniques forreducing the scale of image rasters are known. In this example, thegray-level raster reducing unit 38 determines the above-mentionedrepresentative value as being the mean of the values of the gray levelsof the elements of the block. This gray-level raster at reduced scale isshown in FIG. 4C.

The reduced scale gray-level raster is then transmitted to the half-toneunit 26 where it is put into half-tone form as a binary raster havingthe same reduced scale.

This binary raster is then transmitted to the point matrix memory 22.The data from the memory containing the binary raster is then read tothe buffered memory 24 and processed by the units 30 to 36 in the samemanner as for data in vector form, as discussed above, so as toreconstitute a raster at the original scale.

When the image data is already in the form of a binary raster, it issubjected to scale reduction by a factor of n (e.g., n=2) in a binaryraster reducing unit 40. This reduction is performed in the same manneras in the raster reducing unit 38 for gray-level rasters, except thatthe representative value given to each point in the reduced rastercorresponding to a block of four points in the input binary raster iscomputed in application of the following rules. The representative valuecorresponds to a point to be printed, if:

i) half or more of the points in the blocks of four points in the inputbinary raster are points to be printed; or

ii) only one point in the block is a point to be printed and the blockis the N.m-th block in a run of not-necessarily consecutive blocks eachcontaining only one point to be printed, using any systematic method ofcounting blocks, where N is a run of integers 1, 2, 3, . . . , and wherem is a predetermined integer, preferably equal to 4, i.e. to the squareof the scale reduction factor.

In all other cases, the representative value corresponding to a blankpoint.

FIG. 5B is a diagram showing the binary raster of FIG. 5A after thescale reduction in application of the above rules is performed.

The four-point blocks in the input binary raster for which only onepoint has binary value “1” are identified in FIG. 5A (by ringednumbers). In this example, these blocks are counted by scanning the rowssuccessively starting from the first. Counting is restricted to thepattern shown and is not re-started for each row or each column,although that may be done in practice. In general, any counting methodcan be used for performing above rule ii), provided that the ruleensures that one isolated point out of every m isolated points isprinted.

In FIG. 5B, the points in the binary raster at reduced scale that havebeen given a “1” value by rule ii) are identified by the symbol (0→1).They correspond to each block identified in FIG. 5A by a count valuewhich is a multiple of m=4.

Counting is performed by an isolated point counter unit 42 (FIG. 7).

Rule ii), which is optional in the context of the present invention,serves to identify isolated points in a pattern and to retain one pointin four isolated points. Application of this rule is advantageous when adistribution of isolated points causes half-tones to be shown.

The binary raster reduced in this way by the unit 40 is transmitteddirectly to the point matrix memory 22. Thereafter, data processingwhich is identical to that performed in the two preceding cases isperformed.

For a better understanding of the result of printing in ink economy mode(or draft mode) in accordance with the invention. FIG. 8A shows an imagethat is printed in high quality mode with an ink-jet printer startingfrom input image data in the form of a binary raster, while FIG. 8Bshows the same image printed in ink-economy mode after application ofabove rules i) and ii) according to the present invention. It will beobserved that the half-tones (obtained by space modulation of points tobe printed or “dithering”) are well preserved, in spite of the largereduction in the quantity of ink used.

The above-described examples of the method of printing with printeconomy are just as applicable to color printing as to monochromeprinting. In color printing, each color component, and optionally theblack component, of a pattern is processed separately and in the samemanner for scale reduction, raster computation, and insertion of blankimage points.

It should also be observed that the scale reduction factor can be anyreal number, not necessarily an integer. The invention makes it possibleto use known reduction algorithms that enable a reduced-scale raster tobe computed over a quasi-continuous range of reduction factors startingfrom image data in vector or raster form. More generally, the presentinvention can be implemented with any image reduction technique, itbeing understood that reduction by an integer scale factor, and inparticular a factor equal to two, requires computation that isrelatively simple and fast and provides the desired savings of print andof energy without degrading the information content of the imageunacceptably.

Finally, the point matrix computed from the reduced scale image may beother the binary, e.g. ternary or quaternary, depending on the printingtechnology used. Each point of the matrix may then define a valueselected from a set of discrete values specifying either a number ofdots to be printed in a zone corresponding to a single print point, orelse the size of the dot to be printed, so as to reproduce print pointswith varying intensity.

The invention being thus described it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A method of printing raster images whileeconomizing printing resources, starting from input image correspondingto a predetermined original format, the method comprising the steps of:reducing the input image relative to the original format; on the basisof the reduced image, forming a reduced raster of image points eachdefining either a point to be printed or a blank point of the image;enlarging said reduced raster of image points to return to the originalformat by inserting blank image points in the reduced raster by apredetermined filling procedure; and printing an image on the basis ofthe raster of image points enlarged to the original format.
 2. A methodaccording to claim 1, characterized in that the input image is reducedby a scale factor of n, where n is a real number greater than
 1. 3. Amethod according to claim 2, characterized in that the number n is aninteger number greater than or equal to
 2. 4. A method according toclaim 3, characterized in that the number n is equal to
 2. 5. A methodaccording to claim 3, wherein the raster enlargement step includesadding n-1 blank image point(s) between two points in each row of imagepoints in the reduced raster, and adding n-1 row(s) of blank pointsbetween two rows of image points of the reduced raster.
 6. A methodaccording to claim 2, wherein, when data on the input image is in theform of vectors specified relative to a coordinate plane, the reductionstep includes dividing a definition of the input image by n.
 7. A methodaccording to claim 2, wherein, when data on the input image is in theform of a raster, the reduction step includes forming a reduced rasterin which each point is given a representative value representing thedata in a predetermined neighborhood zone of an input raster and havingtopographical correspondence with said point.
 8. A method according toclaim 7, wherein said neighborhood zone is composed of a block of n×npoints constituted by n contiguous image points in the row direction andn contiguous image points in the column direction of the input raster.9. A method according to claim 7 or 8, characterized in that, when theinput raster is a gray-level raster, said representative value is anaverage of the gray level values in said neighborhood zone.
 10. A methodaccording to claim 7, wherein, when the input raster is a binary rasterin which each point has one or the other of two values defining a pointto be printed and a blank point, said representative value is computedas a function of the points of said neighborhood zone.
 11. A methodaccording to claim 8, wherein, when the input raster is a binary rasterin which each point has one or the other of two values defining a pointto be printed and a blank point, said neighborhood zone is a 2×2 pointblock and said representative value corresponds to a point to be printedif half or more of the points of the block are points to be printed, andcorresponds to a blank point otherwise.
 12. A method according to claim8, wherein, when the input raster is a binary raster in which each pointhas one or the other of two values defining a point to be printed and ablank point, said neighborhood zone is a 2×2 point block and saidrepresentative value corresponds to a point to be printed if: half ormore of the points of the block define a point to be printed; or onlyone point of the block is a point to be printed and the block is theN.m-th block in a run of not-necessarily consecutive blocks eachcontaining a single point to be printed, where N is a run of integers 1,2, 3, . . . , and where m is a predetermined integer such that thereduced raster includes one point to be printed for each set of m inputraster blocks containing only one point to be printed in each block. 13.A method according to claim 12, characterized in that m is equal to 4.14. A method according to claim 1, wherein, for color printing, saidreducing, forming and enlarging steps are performed for each colorcomponent.
 15. An apparatus for printing rasterized images whileeconomizing printing resources, starting from input image correspondingto a predetermined original format, the apparatus comprising: means forreducing the input image relative to the original format; means forforming a reduced raster of image points on the basis of the reducedimage, each image point defining either a point to be printed or a blankpoint of the image; means for enlarging the reduced raster of imagepoints to return to the original format by inserting blank image pointsin the reduced raiser by a predetermined filling procedure; and meansfor printing an image on the basis of the raster of image pointsenlarged to the original format.
 16. An apparatus according to claim 15,wherein the means for reducing the input image are designed to implementreduction by a scale factor of n, where n is a real number greaterthan
 1. 17. An apparatus according to claim 16, wherein n is an integernumber greater than or equal
 2. 18. An apparatus according to claim 17,wherein n is equal to
 2. 19. An apparatus according to claim 17, whereinthe means for enlarging the raster operates by inserting n-1 blank imagepoint(s) between two points in each row of image points in the reducedraster, and by adding n-1 row(s) of blank points between two rows ofimage points in the reduced raster.
 20. An apparatus according to claim16, wherein, when data on the input image is in the form of vectorsspecified relative to a coordinate plane, the means for reducingoperates by dividing a definition of the input image by n.
 21. Apparatusaccording to claim 16, wherein, when data on the input image is in theform of a raster, the means for reducing operates by forming a reducedraster in which a representative value is given to each pointrepresenting data in a predetermined neighborhood zone of an inputraster and having topographical correspondence with said point.
 22. Anapparatus according to claim 21, wherein said neighborhood zone iscomposed of a block of n×n points constituted by n contiguous imagepoints in the row direction and n contiguous image points in the columndirection of the input raster.
 23. An apparatus according to claim 21,wherein, when the input raster is a gray-level raster, saidrepresentative value is an average of the gray level values in saidneighborhood zone.
 24. An apparatus according to claim 21, wherein, whenthe input raster is a binary raster in which each point has one or theother of two values defining a point to be printed and a blank point,said representative value is computed as a function of the points ofsaid neighborhood zone.
 25. An apparatus according to claim 22, wherein,when the input raster is a binary raster in which each point has one orthe other of two values defining a point to be printed and a blankpoint, said neighborhood zone is a 2×2 point block and saidrepresentative value corresponds to a point to be printed if half ormore of the points of the block are points are to be printed, andcorresponds to a blank point otherwise.
 26. An apparatus according toclaim 22, wherein, when the input raster is a binary raster in whicheach point has one or the other of two values defining a point to beprinted and a blank point, said neighborhood zone is a 2×2 point blockand said representative value corresponds to a point to be printed if:half or more of the points of the block define a point to be printed; oronly one point of the block is a point to be printed and the block isthe N.m-th block in a run of not-necessarily consecutive blocks eachcontaining a single point to be printed, where N is a run of integers 1,2, 3, . . . , and where m is a predetermined integer such that thereduced raster includes one point to be printed for each set of m inputraster blocks containing only one point to be printed in each block. 27.An apparatus according to claim 26, wherein m is equal to
 4. 28. Anapparatus according to claim 15, wherein, for color printing, said meansfor reducing the input image, said means for forming the reduced rasterof points, and said means for enlarging the reduced raster operate oneach color component.
 29. An apparatus for printing rasterized imageswhile economizing printing resources, starting from input imagecorresponding to a predetermined original format, the apparatuscomprising: a first section reducing the input image relative to theoriginal format; a second section forming a reduced raser of imagepoints on the basis of the reduced image, each image point definingeither a point to be printed or a blank point of the image; a thirdsection enlarging the reduced raster of image points to return to theoriginal format by inserting blank image points in the reduced raster bya predetermined filling procedure; and a fourth section printing animage on the basis of the raster of image points enlarged to theoriginal format.